Uh-oh... That clone() will do more, actually - it will clone a bunchof vfsmounts. What happens if you create a separate namespace for thefirst thread, so that the second one would not have our vfsmount toplay with?

Alternatively, what if the second thread is doing mount --bind cgroup foo umount fooin a loop?

Another one: does turning the umount in the first thread into umount -laffect anything?